In a boiling water reactor, a plurality of fuel assemblies are arranged and loaded in a square lattice shape in a core provided in a reactor pressure vessel. The fuel assemblies loaded into the core reside in the core for a predetermined period, are taken out of the reactor after reaching a predetermined burnup degree, and are exchanged for new fuel assemblies.
In the core of the related art, it is common to operate the core by a fuel assembly having the maximum number of residence cycles in the core being loaded into the outermost periphery of the core and fuel assemblies having different numbers of residence cycles in the core by including a new fuel assembly and the fuel assembly with the maximum number of residence cycles in the core are dispersedly loaded into an inside of the outermost periphery of the core.
In a method for operating the boiling water reactor of the core of the related art, in a case where the boiling water reactor is decommissioned, a fuel assembly newly loaded during fuel exchange before the decommissioning has a problem that fuel cost to power generation amount is increased by a large amount of unreacted fissile uranium of which the number of residence cycles in the core is shorter than that during normal operation being remained.
For example, in PTL 1, fuel cost of a four-batch core is calculated and PTL 1 describes that the fuel cost is significantly deteriorated in three cycles before the decommissioning and, in particular, the fuel cost is twice or more in a cycle immediately before the decommissioning. Therefore, PTL 1 discloses a method for alleviating deterioration in fuel cost by loading a fuel assembly having a smaller inventory of uranium heavy metal than that of a normally used fuel assembly during the final fuel exchange immediately before the decommissioning. Specifically, in order to alleviate the deterioration in the fuel cost in the boiling water reactor before the decommissioning, a configuration in which a fuel assembly having increased the number of water rods to be disposed is loaded into the core during the final fuel exchange prior to the decommissioning is disclosed. In addition, instead of increase in the number of water rods to be disposed, a configuration in which fuel assemblies in which fuel rods not filled with fuel pellets are disposed or fuel assemblies in which fuel rods are reduced in diameter are loaded is described.